A refrigerant boiling or condensing sits at ONE temperature for a given pressure — its saturation temperature, $T_{sat}$, read off a P-T chart or the collar of the gauge. While liquid and vapour are both present, the thermometer will not move. So the only way to know whether the last drop has boiled, or the first drop has condensed, is to compare a measured line temperature with the saturation temperature the pressure implies. That comparison has two names, one for each side of the system.

Superheat lives on the low side, at the compressor suction: $\mathrm{SH} = T_{suc} - T_{sat}$, where $T_{suc}$ is the temperature measured on the suction line in °C and $T_{sat}$ is the saturation temperature at the suction pressure, also in °C. The answer is a temperature DIFFERENCE, so it is quoted in kelvins. Positive superheat is proof the vapour is dry — and dry vapour is the only thing a compressor may be asked to swallow. Zero superheat means liquid is arriving, and liquid does not compress.

Subcooling lives on the high side, at the condenser outlet: $\mathrm{SC} = T_{sat} - T_{liq}$, with $T_{sat}$ the saturation temperature at the head pressure and $T_{liq}$ the temperature measured on the liquid line. Note the order has flipped, and it flipped for a reason: subcooled liquid is COLDER than saturation, so the saturation temperature goes first to keep the answer positive. Subcooling is the insurance that solid liquid — not a bubble of flash gas — reaches the expansion valve.

Both numbers are gaps, never states. A negative superheat or a negative subcooling is not a small reading; it is a wrong one, or a machine in trouble. And both need a pressure AND a temperature: a thermometer alone on a refrigerant line tells you almost nothing.